clang-rename: add new -force option
The use-case is when renaming a widely used name, like a lower-level class in a codebase and clang-rename is simply invoked for each translation unit based on the compile database. In this case it's not interesting to show errors: not finding the symbol means there is simply nothing to do.
Reviewed By: klimek
Differential Revision: https://reviews.llvm.org/D32403